Wind tunnel calibration of drift samplers

摘要

For drift studies, active samplers can provide better sampling efficiency especially for smaller droplets. If properly calibrated, they can provide absolute measurement of time-integrated drift flux. A small open wind tunnel (nozzle type) with a 30x30cm exit port was designed to provide laminar flow between 1 and 5 m/s. The droplet source utilized satellite droplets from a uniform droplet generator based on the liquid jet instability. Droplets of diameter down to 25 um can be routinely produced at aknown rate (droplets per second). This droplet stream was injected in the wind tunnel flow upstream of the samplers being calibrated. Three types of samplers were used: stationary cylindrical target (1.6 mm dia), a Rotorod with sampling rods 1.6 mm in diameter and a rotating rod (Rotobar) 1.6 mm in diameter revolving in a plane perpendicular to the air flow. Sampling efficiency was assessed using food dyes collected on the rods using light transmittance measurements. Results showed that impaction efficiency for the static rod compared reasonably well with published correlation of catch efficiency versus Stokes number. For the active samplers (Rotorod and Rotobars), these correlations need to be adjusted with sampler type and air speed.